1. Field of the Invention
The present invention relates to a communication control method in a mobile communication system and a base station and a program record medium used therefor, and in particular, to the communication control method of a channel from the base station to a mobile station on soft handover in a CDMA (Code Portion Multiple Access) cellular mobile radio communication system.
2. Description of the Prior Art
One of the cellular mobile communication systems is an HSDPA (High Speed Downlink Packet Access) mode, which is a technology for transmitting packeted data by using a high-speed downlink shared channel (downlink-specific) called HS-PDSCH (High Speed-Physical Downlink Shared Channel) in the case of transmitting to a mobile station large amounts of data (such as the data of a static image and a dynamic image) from a communication network via a base station. It is a mode wherein, in the case where the large amounts of data must be simultaneously transmitted to a plurality of mobile stations, this high-speed downlink shared channel called HS-PDSCH is shared time-wise (time sharing) to use this one high-speed channel by sharing.
When the mobile station approaches a boundary of a cell in such a cellular mobile communication system in the HSDPA mode, the mobile station is in a state capable of communication by setting up channels with a plurality of base stations having the cells near this boundary as their service areas, which is called a soft handover state. At the time of this soft handover, for example, channels are connected between a certain mobile station (MS) 10 and two base stations (BS) 1 and 2 as shown in FIG. 1.
As for these channels, there are two types as shown in FIG. 1, where one type is a downlink-specific common pilot channel called CPICH (Common Pilot Channel), indicated as a CPICH 1 and a CPICH 2 corresponding to the base stations 1 and 2 respectively. In addition, the other type is a dedicated (physical) channel for uplink/downlink called DPCH (Dedicated Physical Channel) and includes various communication data such as control information and voice. It is also indicated as a DPCH 1 and a DPCH 2 corresponding to the base stations 1 and 2 in FIG. 1. Moreover, in FIG. 1, a PDSCH is the above-mentioned large-capacity data channel, which is a downlink-specific shared data channel.
Moreover, definitions of such various channels are disclosed in the 3GPP (3rd Generation Partnership Project) Specifications TS25.211V3.0.0 (1999-10).
It adopts a method wherein, on the soft handover, a plurality of base stations 1 and 2 simultaneously transmit the DPCH to a mobile station 10, and the PDSCH that is the data channel is transmitted by just one base station (only the base stations 1 in FIG. 1) so as to control transmitting power of this PDSCH in accordance with the DPCH, which is disclosed in the 3GPP Report TR25.841V4.0.0 (2000-12).
According to this report, transmitting power PPDSCH from the base station of the PDSCH is determined as follows.PPDSCH=PDPCH1·A·C  (1)Here, the transmitting power of the DPCH1 is PDPCH1, and A is a constant and selected as A=1 for instance. And C is defined as follows.PCPICH1>PCPICH2In this case, C=Cprimary (=2 to 3 dB), and ifPCPICH1<PCPICH2,it is C=Cnon-primary (=4 to 6 dB). In addition, PCPICH1 and PCPICH2 are the received power of the CPICH 1 and CPICH 2 in the mobile station.
In another mobile communication system for performing data communication from the base station to the mobile station by using the PDSCH, the transmitting power of this PDSCH is not controlled but is fixed, and instead, it is possible to select a plurality of transmission modes of different modulating methods or encoding methods. As for the modulating methods, there are QPSK (Quadrature Phase Shift Keying) capable of transmitting 2 bits (4 values) by 1 symbol, 16QAM (16 Quadrature Amplitude Modulation) capable of transmitting 4 bits (16 values) by 1 symbol, 64QAM (64 Quadrature Amplitude Modulation) capable of transmitting 6 bits (64 values) by 1 symbol and so on. In addition, as for the encoding methods, there are convolutional encoding of an encoding ratio 1/2 for performing encoding by 2 bits per information bit, the convolutional encoding of an encoding ratio 1/2 for performing encoding by 3 bits per information bit and so on.
FIG. 11 shows an example of selection of the transmission modes of different modulating methods. In FIG. 11, for instance, the transmission mode is selected so that, a distant from a base station 51 to a mobile station 52 is shorter, a transmitting speed becomes to be faster. To be more specific, the base station 51 selects the transmission modes of 64QAM, 16QAM and QPSK in increasing order of distance to the mobile station 52.
While the DPCH is used together with the PDSCH in order to transmit and receive control information on the data transmission by the PDSCH, there is a method in the past, as that of selecting the above-mentioned transmission modes of 64QAM, 16QAM and QPSK, wherein a range of the transmitting power in the base station of the DPCH of the downlink is determined in advance so as to select the transmission mode according to that transmitting power.
In this case, the mobile station measures receiving quality of the DPCH from the base station, compares the measured value to a reference value, and notifies the base station of increase or decrease in the transmitting power of the DPCH so that the measured value becomes close to the reference value. The base station increases or decreases the transmitting power of the DPCH according to that notification, and also selects the transmission mode according to the transmitting power set up by the increase or decrease. For instance, as shown in FIG. 12, it selects the transmission modes of the 64QAM, 16QAM and QPSK in increasing order of the transmitting power to be set up.
As for the method of determining the transmitting power of the PDSCH shown in the above-mentioned equation (1), it is a two-step control method wherein the transmitting power of the PDSCH is determined by rendering the C of the equation (1) as Cprimary if, of a plurality of base stations in the soft handover state, the base station setting up the PDSCH is one having higher received power of the CPICH in the mobile station (Primary), and by rendering the C as Cnon-primary otherwise, resulting in a problem that continuous and elaborate control cannot be exerted. To be more specific, there is a fault that it is not possible to maintain the receiving quality of the PDSCH at a fixed level regardless of a position of the mobile station 10 on the soft handover.